RPython is certainly not perfect, and the PyPy/RPython community is also very much aware of RPython being something that developed over time. But, RPython was from the start 'designed' as a language for implementing dynamic languages. The problem is that it took multiple iterations to identify meta-tracing as a foundation that works. And those iterations left some cruft here and there.
Compared to the Java-based experiments I did, sometimes I miss the ability to be explicit about types, and have to use RPython's assertions instead, which is a clumsy way of telling RPython about types. But overall, RPython isn't the worst toolchain I have ever worked with. And, typically, if the error messages are really bad, the community is very helpful and eager to improve them.
Compared to the Java-based experiments I did, sometimes I miss the ability to be explicit about types, and have to use RPython's assertions instead, which is a clumsy way of telling RPython about types. But overall, RPython isn't the worst toolchain I have ever worked with. And, typically, if the error messages are really bad, the community is very helpful and eager to improve them.